Crop water use, also called evapo-transpiration or ET, is an estimate of the amount of water used or “transpired” by the plants, and the amount of evaporation from the soil surface around the plants.

A plant's water use changes with a predictable pattern from germination to maturity. All crops have a similar water use pattern. However, crop water use can vary with each growing season due to changes in climatic variables (high temperature, amount of sunlight, humidity, wind) and soil differences between fields, such as soil water holding capacity.

Research has produced accurate estimates of crop water use. The water use figures in the table help illustrate again that sunflower is relatively drought-resistant compared to other crops. Studies have shown that in addition to the low water requirement, sunflower is relatively resistant to the effects of short water stress periods. The water use efficiency (translated as seed production per inch of water use by the plant when water is limiting) represents the response of adequately fertilized crops.

Sunflower performs well in dry conditions due to its deep, aggressive root system. The seasonal water needs of sunflower is typically around 22 inches of moisture under good moisture conditions. In general, yield is directly related to water availability. A conservative estimate is that each inch of available water will produce about 100 pounds of seed. Others have estimated sunflower yield response to available water at 150 lbs of seed per acre produced for every inch of water use after the first seven inches. Water is most critical to sunflower at stand establishment and growth stages R-4 to R-8, or just prior to flowering through seed filling.